Vince,
Remember when I mentioned asymmetrical rails back when we were talking about current sources for the SOZ? Same thing Nelson's talking about.
Okay, here's the deal...
You're familiar with the idea of a voltage regulator, right? An ideal voltage regulator (as opposed to a real world one) will deliver any arbitrary amount of current from 0 amps to infinite amps while maintaining an absolutely steady voltage. That's why it's a *voltage* regulator--the voltage remains constant, no matter what, but the current varies.
Now, a current source has another name: current regulator. What's a current regulator do? (You can already see where I'm going, I'll bet.) Locks the current...and lets the voltage vary. Kind of like an upside-down voltage regulator.
Suppose we were to say we wanted a steady 1 amp. We then build a current source with this in mind. How would it behave? If you give a 1A current source a 1 ohm load, it will develop exactly 1V across the load. Simple application of Ohm's Law: I*R=E...1A*1 ohm=1V. But what happens if you give it a 2 ohm load? It will do whatever it has to do to force 1A through the load. In this case, it will develop 2V of output. 1A*2 ohms=2V. An ideal current source could develop 1kV across a 1k resistor, simply because you told it to deliver 1A, no matter what.
So let's consider what happens if you put one into a differential circuit (i.e. the SOZ). When there's no signal, each device will conduct half of the current. If a balanced signal is applied, one device will conduct more at the same time as the other is conducting less, but always such that the instantaneous total is 1A, regardless of whether the ratio is 90/10, 70/30, or 20/80 at any given moment. The current regulator *will* continue to push one amp through the load.
If you put an unbalanced signal in, the device receiving the signal will behave in accordance with the signal--just like you'd expect. But...that current source is a stubborn l'il critter. It *will* have its way. As the device receiving the signal decreases conductance, the rest of the current gets forced through the other side, willy-nilly, thus creating a signal through that device, even though there wasn't any input at the gate (note that the same principles hold for tubes and bipolars, I'm just using MOSFETs because that's how Nelson laid out the SOZ). An important feature to note here is that the signal through the other side of the differential is *out of phase* from the original signal, creating a balanced signal, where none was before. As the first side goes negative, the second side goes positive, and vice versa.
Due to the fact that current sources vary voltage so easily, this leads us to the possibility of asymmetrical rails for the SOZ. If you program a current source for 1A and give it, say, a -30V rail, it will develop any voltage it has to in order to supply 1A to the load and--this is the keen part--it's self adjusting! If you give it a -10V rail, it will adjust. If you give it a -300V rail, it will adjust. All because a current source excels at changing voltage whilst maintaining current.
Since the SOZ circuit doesn't need a whole lot of negative voltage underneath the MOSFET's source pin (we're assuming N-channel devices, here), you can get away with trimming the fat. Leave some wiggle room for the signal, add a volt or two for the current source itself, add bias, and stir, and presto! you've got a recipe for an amp with asymmetrical rails. Set things up to where you've got 10-15V under the tail of the thing, and you can put any voltage you like on top, whether it's 10, 20, or 50V. But the lower rail can stay the same.
There's one caveat: Real world current sources ain't perfect. They suffer from capacitance and other woes that slow them down, making them somewhat less than the ideal solution.
Grey
P.S.: And don't nobody start griping about the difference between current sources and current sinks, and such. I'm using the term current source in the vernacular sense because that's what everybody calls 'em. Okay? Start messin' with me about nomenclature an' Santa Clause won't bring you that order of MOSFETs you want for Christmas!